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[libcxxabi] Use __LDBL_MANT_DIG__ for configuring demangling of long doubles #134976

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Merged
merged 2 commits into from
Apr 11, 2025
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[libcxxabi] Use __LDBL_MANT_DIG__ for configuring demangling of long doubles #134976

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[libcxxabi] Use __LDBL_MANT_DIG__ for configuring demangling of long …
mstorsjo Mar 12, 2025
8821bca
Actually check the last case, add an #error if the size is unhandled
mstorsjo Apr 9, 2025
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14 changes: 6 additions & 8 deletions libcxxabi/src/demangle/ItaniumDemangle.h
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Original file line number Diff line number Diff line change
Expand Up @@ -5741,14 +5741,12 @@ struct FloatData<double>
template <>
struct FloatData<long double>
{
#if defined(__mips__) && defined(__mips_n64) || defined(__aarch64__) || \
defined(__wasm__) || defined(__riscv) || defined(__loongarch__) || \
defined(__ve__)
static const size_t mangled_size = 32;
#elif defined(__arm__) || defined(__mips__) || defined(__hexagon__)
static const size_t mangled_size = 16;
#else
static const size_t mangled_size = 20; // May need to be adjusted to 16 or 24 on other platforms
#if __LDBL_MANT_DIG__ == 113
static const size_t mangled_size = 32;
#elif __LDBL_MANT_DIG__ == 53
static const size_t mangled_size = 16;
#else // __LDBL_MANT_DIG__ == 64
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I think it'd be better to check the mantissa here as well and #error if there is a new size. Otherwise we might be silently wrong on some platforms.

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That sounds like a good idea, will fix.

However - I realized that while we don't support building libcxxabi with compilers that aren't GCC compatible, we also do copy this file into llvm/include/llvm/Demangle/ItaniumDemangle.h, which is built as part of the host build of LLVM - and that can be built with MSVC as well. (And MSVC doesn't define any of the arch defines that we defined before either.)

MSVC only has long double == double for all currently known architectures (and clang-cl matches this), so I can add an || defined(_MSC_VER) for that.

static const size_t mangled_size = 20;
#endif
// `-0x1.ffffffffffffffffffffffffffffp+16383` + 'L' + '\0' == 42 bytes.
// 28 'f's * 4 bits == 112 bits, which is the number of mantissa bits.
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4 changes: 0 additions & 4 deletions libcxxabi/test/test_demangle.pass.cpp
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Expand Up @@ -13,10 +13,6 @@
// dd8b266ef.
// UNSUPPORTED: using-built-library-before-llvm-20

// Android's long double on x86[-64] is (64/128)-bits instead of Linux's usual
// 80-bit format, and this demangling test is failing on it.
// XFAIL: LIBCXX-ANDROID-FIXME && target={{i686|x86_64}}-{{.+}}-android{{.*}}

// XFAIL: win32-broken-printf-a-precision

#include "support/timer.h"
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